Stabilize streaming tests in test_decoding.

We have some streaming tests that rely on the size of changes which can
fail if there are additional changes like invalidation messages by
background activity like auto analyze. Avoid such failures by increasing
autovacuum_naptime to a reasonably high value (1d).

Author: Dilip Kumar
Backpatch-through: 14
Discussion: https://postgr.es/m/1958043.1650129119@sss.pgh.pa.us

contrib/test_decoding/logical.conf

 wal_level = logical
 max_replication_slots = 4
 logical_decoding_work_mem = 64kB
+autovacuum_naptime = 1d